The most common dietary deficiency worldwide is iron, affecting half a billion persons. However, this problem affects women and children more. A growing child is increasing the red blood cell mass, and needs additional iron. Women of reproductive age who are menstruating require double the amount of iron that men do, but normally the efficiency of iron absorption from the gastrointestinal tract can increase to meet this demand. Also, a developing fetus draws iron from the mother, totaling 200 to 300 mg at term, so extra iron is needed in pregnancy. An infant requires formula with 4 - 12 mg/L of iron. Iron in breast milk is more readily absorbed.